1. Field of the Invention
The present invention relates to a method of fabricating an optically coupled device having an assembly of a photoemitter that emits light of a predetermined level according to a flow of input current and a photodetector that receives the light from the photoemitter to provide output current.
2. Description of the Background Art
An optically coupled device is conventionally fabricated using a photoemitter and a photodetector. In the fabrication of an optically coupled device, there is variation in the characteristics of the photoemitter as well as in the photodetector. By the synergistic effect of the variation in both the characteristics of the photoemitter and the photodetector, the variation in the characteristics of the optically coupled device is significantly increased.
A plurality of photoemitters are fabricated from one semiconductor wafer. Also, a plurality of photodetectors are fabricated from one semiconductor wafer. The characteristics of the plurality of photoemitters are tested for every one wafer in preparation for the fabrication of an optically coupled device. Also, the characteristics of the plurality of photodetectors are tested for every one wafer in preparation for the fabrication of an optically coupled device.
Only a photoemitter whose variation range in characteristics is smaller than the entire variation permissible range of characteristics of a photoemitter based on the characteristic test of each wafer is used in the fabrication of an optically coupled device.
Also, only a photodetector whose variation range in characteristics is smaller than the entire variation permissible range of characteristics of a photodetector based on the characteristic test of each wafer is used in the fabrication of an optically coupled device.
Thus, measures are taken so that the variation range in characteristics of an optically coupled device is below a desired level as to optically coupled devices whose variation range in characteristics is increased by the synergistic effect of the variation in characteristics of both the photoemitter and the photodetector.
In the fabrication of a conventional optically coupled device, various processes such as the photoemitter and photodetector fabrication process, the photoemitter and photodetector test process, the optically coupled device assembly process (die-bonding, wire-bonding, precoating, molding and the like), the optically coupled device test process, and the packaging process are respectively controlled by an independent computer.
Due to the synergistic effect of variation in characteristics of a photoemitter and a photodetector, the variation in characteristics of an optically coupled device will become greater than the entire variation in characteristics of a photoemitter and the entire variation in characteristics of a photodetector.
In the case where a certain photoemitter and a certain photodetector are combined, there is a possibility of the assembled optically coupled device having a value of characteristics outside the permissible range despite each certain photoemitter unit and photodetector unit having a characteristic variation within the permissible range. In such a case, the yield of the optically coupled device is degraded.
Furthermore, in order to improve the yield of the optically coupled device, it is necessary to set the variation in the characteristics of the photoemitter unit and the photodetector unit to be with in an extremely small range taking into account the synergistic effect of the variation in characteristics of both the photoemitter and the photodetector.
However, if the respective variations in characteristics of the photoemitter unit and the photodetector unit are set to be within an extremely small range, there is a disadvantage that the respective yields of the photoemitter and the photodetector will be degraded.
A specific example of this disadvantage will be described based on a photocoupler including an infrared-emitting diode and i phototransistor.
Referring to FIG. 7, a photocoupler 1 has an infrared-emitting diode 2 die-bonded to a frame 4, and a phototransistor 3 die-bonded to a frame 5. Infrared-emitting diode 2 is molded by a precoat resin 6.
Infrared-emitting diode 2 and phototransistor 3 are disposed facing each other. Frames 4 and 5, infrared-emitting diode 2 and phototransistor 3 are molded by a first mold resin 7 and a second mold resin 8.
When a current is input (referred to as xe2x80x9cIFxe2x80x9d hereinafter: unit [A]) to infrared-emitting diode 2 from a first circuit at the input side as shown in FIG. 8 in photocoupler 1, infrared light is output from infrared-emitting diode 2.
This infrared light is received by phototransistor 3. Upon receiving infrared light, phototransistor 3 conducts a flow of an output current (referred to as xe2x80x9cICxe2x80x9d hereinafter: unit [A]) at a predetermined amplification factor (referred to as xe2x80x9chFExe2x80x9d hereinafter) to a second circuit at the output side.
By the above-described mechanism, photocoupler 1 can transmit an electrical signal from the first circuit to the second circuit with the first circuit at the input side insulated from the second circuit; at the output side. In photocoupler 1, the ratio of IC to IF, i.e., IC/IFxc3x97100 is called the current transmission rate (referred to as xe2x80x9cCTRxe2x80x9d hereinafter: unit [%]).
When an electronic circuit is fabricated using photocoupler 1, the circuitry must be designed taking into account the change in the CTR due to temperature and over time. The designing of circuitry employing photocoupler 1 will become easier as the range of change in CTR is smaller.
The CTR of photocoupler 1 is generally determined by the amount of light (quantity of light) arriving at phototransistor 3 (quantity of light referred to as xe2x80x9cPOxe2x80x9d hereinafter) among the light output from infrared-emitting diode 2 and the hFE of phototransistor 3.
This means that the variation range of CTR is extremely increased due to the synergistic effect of the variation of PO and the variation of hFE. As a result, the yield of photocoupler 1 with respect to CTR will be degraded.
Infrared-emitting diode 2 is fabricated by epitaxial growth for every one batch formed of n wafers. However, the quantity of light of infrared-emitting diode 2 in each wafer of one batch is actually variable, as shown in FIG. 9. This means that the variation in the PO distribution of each of the n wafers is not equal even in the case where epitaxial growth is conducted in the same one batch.
Furthermore, the actual quantity of light of infrared-emitting diode 2 per one batch varies as shown in FIG. 10. The variation in the PO distribution per 1 batch becomes greater than the quantity of light variation of each wafer in one batch.
Therefore, the total variation of the PO distribution of infrared-emitting diode 2 that is used by the manufacturer of photocoupler 1 will become further greater than the quantity of light variation of each wafer in one batch and the quantity of light variation per one batch, as shown in FIG. 11.
The same can be said for the fabrication of a phototransistor. The total variation in the hFE distribution shown in FIG. 14 is greater than the hFE distribution variation of each wafer shown in FIG. 12 and the hFE distribution variation for one batch shown in FIG. 13.
In the present state of affairs, the hFE range of the phototransistor can be specified during fabrication of a photocoupler. However, the PO range of the infrared-emitting diode cannot be specified.
Therefore, a photocoupler 1 having a combination of a phototransistor 3 of a large hFE and an infrared-emitting diode 2 of a large PO will exhibit an extremely large CTR. In contrast, a photocoupler 1 having a combination of a phototransistor 3 of a small hFE and an infrared-emitting diode 2 of a small PO will exhibit in an extremely small CTR.
By the synergistic effect of the variation in characteristics of an infrared-emitting diode and variation in characteristics of a phototransistor, it is difficult to fabricate a photocoupler whose variation range of CTR is within a particular range. As a result, it is difficult to improve the yield of a photocoupler with respect to CTR.
An object of the present invention is to provide a method of fabricating an optically coupled device that can have the yield of optically coupled devices improved.
The optically coupled device fabrication method according to the present invention is directed to a method of fabricating an optically coupled device having an assembly of a photoemitter that issues light of a predetermined level according to a flow of input current and a photodetector that receives the light from the photoemitter to provide an output current.
The fabrication method of an optically coupled device of the present invention includes one or more current transmission rate calculation steps of calculating a current transmission rate that is a ratio of output current to input current under a presumptive assembly condition, and one or more determination steps of determining whether the current transmission rate in each of the one or more current transmission rate calculation steps is a value within a particular range.
When determination is made that the current transmission rate is not within the particular range in each of the one or more determination steps, the presumptive assembly condition is modified to another assembly condition so that the current transmission rate becomes a value in the particular range or approximates the particular range. The one or more determination steps is carried out until the current transmission rate becomes a value in the particular range.
In the method of fabricating an optically coupled device of the present invention, a photoemitter and a photodetector are eventually assembled under an assembly condition where the current transmission rate is within the particular range.
By the fabrication method of the present invention, the current transmission rate can be set to be within a particular range by adjusting the assembly condition, independent of the performance of the photoemitter and the performance of the photodetector. As a result, the yield of optically coupled devices can be improved.
In the case where determination is made that the current transmission rate is a value within the particular range at the first determination step of the one or more determinations steps in the method of fabricating an optically coupled device of the present invention, the photoemitter and the photodetector are assembled under the presumptive assembly condition.
The method of fabricating an optically coupled device of the present invention can include an arriving light quantity measurement step of measuring a quantity of light arriving at the photodetector among the emitted light, and an amplification factor measurement step of measuring an amplification factor that is a ratio of the output current to the quantity of arriving light. In the current transmission rate calculation step, the current transmission rate can be calculated using the measured result of the arriving light quantity measurement step and the measured result of the amplification factor measurement step.
By the present fabrication method, a current transmission rate can be calculated using the arriving light quantity measurement step normally carried out in the product test of photoemitters, and the amplification factor measurement step normally carried out in the product test of photodetectors. Accordingly, increase in the number of steps in the fabrication process of an optically coupled device can be suppressed
In the method of fabricating an optically coupled device of the present invention, the arriving light quantity measurement step can be conducted at a stage before the photoemitter is affixed to a frame, and the amplification factor measurement step can be conducted at a stage before the photodetector is affixed to a frame.
The present fabrication method prevents the disadvantage of the step of affixing a photoemitter to a frame or a photodetector to a frame being wasted when there is a fault in the photoemitter and photodetector that cannot be recovered by modifying other conditions.
In the method of fabricating an optically coupled device of the present invention, the arriving light quantity measurement step can be conducted at a stage after the photoemitter is affixed to a frame, and the amplification factor measurement step can be conducted at a stage after the photodetector is affixed to a frame.
According to the present method, the quantity of arriving light can be measured in a state where the photoemitter is actually affixed to a frame. Therefore, the quantity of arriving light can be measured under a state closer to that of an optically coupled device that is the eventual product, as compared to the case where the quantity of arriving light is measured at a stage before the photoemitter is affixed to the frame.
Furthermore, since the amplification factor is measured under a state where the photodetector is actually affixed to the frame, the amplification factor can be measured in a state closer to the state of the optically coupled device that is the eventual product, as compared to the case where the amplification factor is measured at a stage before the photodetector is affixed to the frame.
In the method of fabricating an optically coupled, device of the present invention, the presumptive assembly condition can be modified by modifying the state of at least one of the frame where the photoemitter is affixed and the frame where the photodetector is affixed.
By the present fabrication method, modification of the presumptive assembly condition can be controlled more easily, as compared to the case where the presumptive assembly condition is modified by modifying the state of the resin as will be set forth below.
In the method of fabricating an optically coupled device of the present invention, the presumptive assembly condition can be modified by altering the resin having a predetermined translucence, provided between the photoemitter and the photodetector, to another resin having a different translucence.
By the present fabrication method, the presumptive assembly condition can be modified without changing the structure of the optically coupled device, as compared to the case where the pressumptive assembly condition is modified by changing the frame state. This eliminates the possibility of a change in structure affecting the characteristics of the optically coupled device.
In the method of fabricating an optically coupled device of the present invention, the presumptive assembly condition can be modified by altering the length of the resin provided between the photoemitter and the photodetector. This length is referenced to the direction connecting the photodetector and the photoemitter.
According to the present fabrication method, the assembly condition can be modified by a more simple operation, as compared to the case where the presumptive assembly condition is modified by changing the resin having a predetermined translucence to another resin of a different translucence or the case where the presumptive assembly condition is modified by altering the frame state.
The photoemitter may be any of a semiconductor infrared photoemitter, a semiconductor visible light photoemitter, and a semiconductor laser device.
The photodetector may be any of a photodiode, a phototransistor, a photo Darlington transistor, a phototriac, a photothyristor, a photoMOS (Metal Oxide Semiconductor), and a photoIC (Integration Circuit).
The optically coupled device may be any of a photocoupler, a phototriac coupler, a photothyrister coupler, and a photointerruptor.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.